This invention relates generally to an active circuit for terminating a transmission line.
Computer and communications systems frequently use transmission lines to transmit data within and to other systems and subsystems. Transmission lines must be properly terminated to minimize distortion of the transmitted signals. This is particularly important as the operating frequencies of transmission lines are increased to match the higher operating frequencies of the latest integrated circuits. Many systems and integrated circuits currently have operating frequencies in the GHz range.
Terminating networks are used to provide the correct terminating impedance. An off-chip precision resistor can provide transmission line termination. Off-chip resistors require significant circuit board space and increase board complexity and cost. The distance between the off-chip resistors and the transmission line to be terminated can be large enough to cause signal reflection problems. The use of on-chip termination resistors is preferred, but high precision resistors are difficult to form using standard CMOS processes due to manufacturing process and thermal variations. A resistor made by current CMOS processes could have a tolerance range of as much as 30%, which is totally unacceptable for use as a transmission line terminator. Laser trimming of on-chip CMOS resistors is expensive and to be avoided, if at all possible.
The use of active circuits, such as FETs (Field Effect Transistors), to terminate a transmission line is known in the art and there have been many approaches to this problem. It is also known in the art that a FET (a field effect transistor) behaves like a resistor for low currents and voltages, but that the channel resistance of a FET is non-linear. For instance, U.S. Pat. No. 5,422,608 describes xe2x80x9cAdaptive Transmission Line Terminationxe2x80x9d in which a FET terminates a transmission line and an amplifier is used to control the voltage applied to the gate of the FET, thus allowing its resistance to be matched to the impedance of the transmission line.
It is an object of the present invention to provide an active circuit for terminating transmission lines and that may be composed of imprecise, high tolerance components configured to operate as a precision termination whose impedance varies only marginally from the characteristic impedance of the transmission line terminated.
The present invention improves upon previous attempts to provide an active termination circuit for terminating transmission lines with a high degree of accuracy, especially when made on an integrated circuit. This invention uses a reference impedance, which is a series combination of an active resistor and a passive resistor. The active resistor is a CMOS transistor operated as a voltage controlled resistor. A variable voltage is applied to the gate to control the drain to source resistance. The passive resistor is set to a fixed resistance.
The current through the reference impedance is set to a fixed value. A control circuit with a feedback loop senses the voltage across the reference impedance to generate a control voltage to control the resistance of the active resistor and thus control the resistance of the reference impedance.
The terminating impedance and the reference impedance are made as identical circuits formed at the same time on the same integrated circuit and as a result have identical properties. The terminating and reference impedances are designed to have the same impedance and are controlled by the same control voltage generated by a feedback loop.
An alternate embodiment of the invention does not use a reference impedance. An active resistor and a passive resistor in series form the terminating impedance circuit. A control circuit senses the voltage on the transmission line and adjusts the gate voltage to control the active resistor to terminate the transmission line with the correct value of resistance.
The present invention has many advantages, which will become clear from the detailed descriptions.